It is desirable to improve combustion of air-fuel mixtures. Particularly when the mixtures are lean, difficulties in ignition may arise. German Patent Disclosure Documents DE-OS No. 23 63 804 and DE-OS No. 28 10 159 describe spark plugs which include a pilot breakdown gap in the spark plug circuit. The pilot breakdown gap controls discharge of a capacitor which is charged to provide spark breakdown energy. Lean air-fuel mixtures sometimes are difficult to ignite. Complete combustion of such lean mixtures which, upon combustion, will be low in environmentally undesirable exhaust, require substantial ignition energy. Conventional ignition systems, designed to be used with fuel-air mixtures which are richer than those which may be applied, may supply insufficient spark energy to the spark plug. Any one sparking event can be characterized by a plurality of individual phases. During the first or breakdown phase of the spark, the amount of energy changed from electrical to thermal, that is, to igniting energy, is a maximum. The remaining energy then results in an arc discharge through the plasma formed upon the original breakdown, with a subsequent glow discharge. Burning of the spark plug electrodes occurs especially during the plasma or second phase, rather than during the initial breakdown phase of the spark. It is, therefore, desirable to provide as much energy as possible during the initial breakdown phase, and to reduce the time during which the arc plasma and the glow phase extend, so that the wear and tear on the electrodes of the spark plug is reduced and hence the operating life as well as the time between "tune-ups" of the engine can be extended.